The present invention relates to a converter having an improved control operation on a primary side.
FIG. 8 is a circuit diagram showing a conventional converter.
Referring to FIG. 8, reference numeral 1 denotes an input DC power source; 2, an input capacitor; 3, a starting resistor; 37, a clamp circuit constituted by diodes 4 and resistor 5 and a capacitor 6; 9, a voltage transformer; 10, a rectification diode; 11, a smoothing capacitor; 12, a load; 29, a control resistor; 30, a diode; 31, a speed-up capacitor; 32, a switching transistor; and 38, a current control circuit constituted by a resistor 15 and a transistor 16; 39, a control circuit constituted by a resistor 33, a variable shunt regulator 34, resistors 35 and 36, and a diode 18, and a capacitor 19.
The operation of the converter will be described below.
When a current from the input DC power source 1 is supplied in the converter, a base current is supplied to the switching transistor 32 through the starting resistor 3, thereby starting the operation of the converter. When the switching transistor 32 is turned on, a voltage is applied to a primary winding N.sub.P of the voltage transformer 9, and a voltage is induced from a control winding N.sub.R of the voltage transformer 9 proportionally to the turn ratio of the control winding N.sub.R. A base current is supplied to the switching transistor 32 through the control resistor 29 to keep the switching transistor 32 in an ON state, and an energy is stored in the voltage transformer 9. When the switching transistor 32 is saturated at a point, the switching transistor 32 is turned off, and a counter electromotive force is generated across the base and emitter of the switching transistor 32 to rapidly turn the converter off. Therefore, the energy stored in the voltage transformer 9 is supplied to the load 12 through the rectification diode 10 and the smoothing capacitor 11.
In the control circuit 39, an output potential is generated proportionally to a turn ratio of the control winding N.sub.R to a secondary winding N.sub.s of the voltage transformer 9 by the diode 18 and the capacitor 19 across the two terminals of a series circuit constituted by the resistors 35 and 36. The base current of the switching transistor 32 is controlled through the variable shunt regulator 34 by a voltage obtained by dividing an output potential which is generated across the two terminals of the series circuit by the resistors 35 and 36. Thus, the switching transistor 32 is turned on/off, and an output voltage is stabilized.
The current control circuit 38 turns the transistor 16 on using a connection-point potential between the resistor 15 and the switching transistor 32 as a detection potential to reduce a base current flowing through the switching transistor 32.
According to the above conventional converter, however, since a bipolar transistor is used as the switching transistor 32, an increase in switching speed is limited by a storage time of the bipolar transistor, and the converter cannot be formed to be compact.
In addition, in the conventional converter, in a case wherein a control current which can be supplied from a control voltage detector is small and the variable range of the current is narrow (e.g., when a shunt regulator is used as the control voltage detector), when a variable range of a current flowing through the impedance circuit is increased due to a wide input voltage variable range of the converter, or when a base current variable range of the switching transistor is increased due to a wide load variable range of the converter, a sufficient control current cannot be supplied to the converter when a high input voltage and a light load are applied to the converter. Therefore, the output voltage of the converter cannot be controlled, and the output voltage is abnormally increased. On the contrary, when the converter is arranged to have a sufficient control current, the efficiency of the converter is degraded.